rebase: check whether the rebasestate exists or not a bit early
Converted the else part into `if True` because that part will soon be under a
except part.
Differential Revision: https://phab.mercurial-scm.org/D3876
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state